The present invention relates generally to the bevelling or grooving of an ophthalmic lens.
It is known to retain an ophthalmic lens in the rim or surround of a spectacle frame, by providing the rim or surround with an annular groove, commonly called a bezel, and by providing on the peripheral edge of the ophthalmic lens, after the latter has been trimmed to the contour of the said rim or surround, a rib or bevel, usually of triangular cross-section, which is arranged to be engaged in the said groove.
Alternatively, it is appropriate to form a groove in the lens when the particular rim or surround of the spectacle frame possesses a tab and/or a wire for retaining the lens.
Simply for the sake of convenience, the following description will most often be confined to the bevelling necessary for forming a rib or bevel in the lens, although the process of the invention be equally used to form a groove.
The bevelling and the preceding trimming operations are usually carried out on a grinding machine equipped for this purpose with at least one bevelling wheel.
On such a grinding machine, the opthalmic lens to be bevelled is maintained by means of its edge in contact with the bevelling wheel, and it is rotated on itself about an axis parallel to the axis of the wheel.
It is, of course, important that the bevel formed on the ophthalmic lens should be accurately on its edge between the ridges of its periphery.
In practice, in order to take into account both the intrinsic curvature of such an ophthalmic lens and the possible variations in its thickness, particularly when a lens of continuously variable focal power, called a progressive lens, is concerned, as well as the "meniscus effect" which the rim or surround, in which it is to be mounted, moreover possesses in its own right, that is to say the intrinsic curvature of this rim or surround, it is necessary to displace the lens parallel to its axis during its rotation relative to the bevelling wheel, in such a way that its point of contact with the wheel follows a suitable path between the ridges of its periphery.
In other words, it is necessary to provide a capability of relative axial displacement between the ophthalmic lens and the bevelling wheel.
The relative axial displacement to be exerted in this way on the ophthalmic lens to be bevelled can be carried out manually.
However, this requires a certain skill on the part of the operator, since the corresponding bevelling of the lens is carried out visually.
Consequently, the results of such an operation are always approximate.
Alternatively, the relative axial displacement of the ophthalmic lens to be bevelled can take place freely by the use of a double-slope bevelling wheel, into the groove of which the entire edge of the lens penetrates so that the lens is therefore automatically continuously centred.
However, such an arrangement is only suitable in practice for ophthalmic lenses which are relatively thin and have a uniform curvature.
For example, when an ophthalmic lens with a thick edge, especially a toric ophthalmic lens, is to be processed, the width which the double-slope bevelling wheel to be used must process quickly becomes prohibitive.
As a corollary to this, when a progressing ophthalmic lens having a variable thickness over its periphery, is to be processed, the bevel to be formed can "exceed" the limits of this periphery in the thinnest zone of the lens in question, or in other words can be inopportunely absent in this thin zone.
Consequently, for the construction of automatic grinding machines designed especially for the processing of such lenses, it has been proposed to control the relative axial displacement of the ophthalmic lens to be bevelled relative to the corresponding grinding wheel such that the point of contact of the ophthalmic lens with this grinding wheel follows a specific path.
Such a control can be systematic, irrespective of the particular characteristics of the lens to be bevelled.
However, whether the axial guidance of the lens is controlled mechanically or by way of a program, only a limited number of possible bevel paths is usually available. For example, three or four such paths may be available, and the operator must select from those that path which seems the most suitable to him, taking into account the particular characteristics of the lens to be processed.
Thus, as before, the operator must necessarily be a specialist.
It has also been proposed to subject the axial guidance of the lens to be processed directly to the particular characteristics of the lens.
This, is the case, for example, in the French Patent Application No. 2,475,446, which describes a first phase corresponding to pre-bevelling carried out by means of a double-slope bevelling wheel, the path followed by the bevel of the lens being recorded by means of detection of the axial displacement of this lens during bevelling, and the axial displacement of the latter taking place freely at this time. In a second phase, during an operation of the finishing bevelling of the said lens, a systematic axial displacement of the latter, corresponding to the previously recorded bevel path, is carried out.
However, as before, such a process is only suitable for lenses which are relatively thin and have a uniform curvature, since if, where the processing of relatively thick or progressive ophthalmic lenses is concerned, pre-bevelling is not carried out in an appropriate way for the reasons mentioned above, the same applies automatically to the final finishing bevelling.